white dots in grease pencil strokes
The dots are transparent and the background shines through. Pretty sure the dots will change color if you turn the background red. 

This happens when a very thick line style is used. GP seems to place lots of short stripes perpendicular to the stroke and the gaps happen when the strokes don't overlap. I tried 2.76 and the line was smooth and even. So some time after that, the way to draw GP has changed. Cranking up 'smooth' seems to help a bit, using a thinner stroke helps, too.

If you need really clean results, converting the strokes to a curve should work, giving it a bit of extrusion and a shadeless material. Sadly GP seems to connect all curves together, so a bit of cleanup is necessary.